Present digital transmission systems try to obtain high spectral efficiencies through gradually more complex modulation formats. The higher spectral efficiency is counterbalanced by the need to increase the transmitted power to obtain a prefixed BER (Bit Error Rate: number of wrong bits to total number of bits ratio) value at the receiver. The power delivered by the transmitter generally is limited by the final power amplifier, which has a greatly nonlinear behaviour. Therefore a serious problem arises with regard to the best exploitation of the nonlinear part of the input-output characteristic of the "channel", including in the latter the final amplifier of the transmitter. At present the problem is faced in one of the following ways (see, e.g., the papers of G. Karam, H. Sari, "Analysis of predistortion, equalization and ISI cancellation techniques in digital radio systems with nonlinear transmit amplifier", IEEE Transaction on Communications, vol. 37, n. 12, Dec. 1989):
1) data predistortion: one tries to modify the constellation used for driving the nonlinear amplifier through a signal such as to obtain the desired constellation at its output; PA1 2) analog signal predistortion: a nonlinear circuit having a characteristic opposite to the one of the above-defined "channel", is inserted in the path of the analog signal; PA1 3) channel equalization and nonlinear cancellation of the ISI: the receive equalizer tries to cancel the interferences connected with nonlinearity from the present signal sample (through a suitable nonlinear combination of pre- and post-cursors); PA1 4) use of "circular" constellations so as to reduce the ratio between the peak power and the average power of the not-filtered signal.
All the above solutions, under special circumstances, can provide unsatisfactory features. In particular the first three are not particularly efficient in the presence of a hard limiter characteristic of the transmitter final amplifier; this last gives rise to gains which are in any event slight and which cannot be sufficient in the case of a reception filter with a very narrow band.